HPDC is a cost-effective and wide-spread method for industrial production of metal components requiring precise dimensional consistency, low dimensional tolerances and where a smooth surface finish is important. Manufacturers in the car industry are now increasingly required to produce near-net-shape aluminum components with a combination of high tensile properties and ductility, and HPDC affords the most economic production method for large-scale quantities of small to medium sized components.
Aluminum alloy castings account for a majority of HPDC castings and are found, for example, in a wide range of automotive parts. In order to avoid discontinuities in the cast component, the molten alloy is injected into the die cavity rapidly enough that the entire cavity fills before any portion of the cavity begins to solidify. Hence, the injection is under high pressure and the molten metal is subject to turbulence as it is forced into a die and then rapidly solidifies. Since the air being replaced by the molten alloy has little time to escape, some of it is trapped and porosity results. Castings also contain pores resulting from gas vapor decomposition products of the organic die wall lubricants and porosity may also result from shrinkage during solidification.
A major drawback of the porosity resulting from the HPDC process is that aluminum alloy castings made from aluminums which ordinarily have the capacity to respond to age-hardening, cannot be artificially aged, that is, they cannot be treated at the high temperatures characteristic of artificial aging conditions. The internal pores containing gases or gas forming compounds in the high pressure die castings expand during conventional solution treatment at elevated temperatures, resulting in the formation of surface blisters on the castings. The presence of these blisters affects not only the appearance of castings but also dimensional stability and in some cases it can negatively impact particular mechanical properties of HPDC components. Specifically, aluminum alloy HPDC cast parts are not amenable to solution treatment (T4) at a high temperature, for example 500° C., which significantly reduces the potential of precipitation hardening through a full temper T6 and/or T7 (equivalently phrased as a combination of temper T4 and T5) heat treatment. It is nearly impossible to find a conventionally processed HPDC component without large gas bubbles.
In Al—Si casting alloys (e.g., alloys 319, 356, 390, 360, 380), strengthening is achieved through heat treatment after casting, with addition of various alloying hardening solutes including, but not limited to, Cu and Mg. The heat treatment of cast aluminum involves a mechanism described as age hardening or precipitation strengthening. Heat treatment (conventional T6 and/or T7 heat treatment) generally includes at least one or a combination of three steps: (1) solution treatment (also defined as T4) at a relatively high temperature below the melting point of the alloy, often for times exceeding 8 hours or more to dissolve its alloying (solute) elements and to homogenize or modify the microstructure; (2) rapid cooling, or quenching into a cold or warm liquid medium after solution treatment, such as water, to retain the solute elements in a supersaturated solid solution; and (3) artificial aging (T5) by holding the alloy for a period of time at an intermediate temperature suitable for achieving hardening or strengthening through precipitation. Solution treatment (T4) serves three main purposes: (1) dissolution of elements that will later cause age hardening, (2) spherodization of undissolved constituents, and (3) homogenization of solute concentrations in the material. Quenching after T4 solution treatment retains the solute elements in a supersaturated solid solution (SSS) and also creates a supersaturation of vacancies that enhances the diffusion and the dispersion of the precipitates. To maximize the strength of the alloy, the precipitation of all strengthening phases should be prevented during quenching. Aging (T5, either natural or artificial aging) creates a controlled dispersion of strengthening precipitates.
With T5 aging, there generally are three types of aging conditions (see FIG. 1), which are commonly referred as underaging, peak aging and over aging. At pre-aging, or an initial stage of aging, Guinier-Preston (GP) zones and fine shearable precipitates form and the casting is considered to be underaged. In this condition, mechanical properties of the casting, for example material hardness and yield strength, are usually low. Increased time at a given temperature or aging at a higher temperature further evolves the precipitate structure increasing mechanical properties such as hardness and yield strength to maximum levels for achieving the peak aging/hardness condition. Further aging decreases the hardness/yield strength and the casting becomes overaged due to precipitate coarsening and its transformation of crystallographic incoherency. FIG. 2 shows an example of aging responses of cast aluminum alloys A356/357 aged at a temperature of 170° C. For the period of aging time tested at giving aging temperature, the castings undergo underaged, peak aged, and overaged stages.
Considering that the conventional HPDC aluminum components inevitably contain internal porosity, artificial aging (T5) becomes a very important step in achieving the desired mechanical properties without causing blistering. The strengthening that results from aging occurs because the retained hardening solutes present in the supersaturated solid solution form precipitates that are finely dispersed throughout the grains and that increase the ability of the casting to resist deformation by slip and plastic flow. Maximum hardening or strengthening may occur when the aging treatment leads to the formation of a critical dispersion of at least one type of these fine precipitates.
In addition, in conventional HPDC processes the cast parts are often slowly cooled to a low temperature, for example, below 200 C, prior to die ejection and quench. This significantly diminishes the subsequent aging potential since the hardening solute solubility decreases significantly with decreasing quench temperature. As a result, the remaining hardening solute, such as Cu and Mg, available in the aluminum matrix for subsequent aging hardening is very limited. Although an alloy may contain 3˜4% Cu in nominal composition, most of the Cu combines with other elements forming intermetallic phases. Without solution treatment, the Cu-containing intermetallic phases will not contribute to age hardening of the material. Therefore, addition of Cu in the current HPDC alloys used in production is not effective in terms of both property improvement and quality assurance.
Typical HPDC aluminum alloys are Al—Si based alloys that contain about 3˜4% Cu. It is generally accepted that copper (Cu) has the single greatest impact of all alloying solutes/elements on the strength and hardness of aluminum alloy castings, both heat-treated and not heat-treated and at both ambient and elevated service temperatures. Cu is known to improve the machinability of alloys by increasing matrix hardness, making it easier to generate small cutting chips and fine machined finishes. On the downside, Cu generally reduces the corrosion resistance of aluminum castings; and in certain alloys and tempers, it increases stress corrosion susceptibility. Cu also increases the alloy freezing range and decreases feeding capability, leading to a high potential for shrinkage porosity.
Further, it has been reported that aluminum alloys with a high copper content (about 3-4%) have experienced an unacceptable rate of corrosion, especially in salt-containing environments. Typical high pressure die (HPDC) aluminum alloys, such as A 380 or 383, which are used for transmission and engine parts, contain 2-4% copper. It can be anticipated that the corrosion issue of these alloys will become more significant, particularly when longer warranty time and higher vehicle mileages are required.
Aluminum alloys have been developed to address some of the known problems, but the castings remain deficient as a whole. For example, Aluminum alloy A380 is a generally age-hardenable alloy with the composition (in wt. %) 9 Si, 3.1 Cu, 0.86 Fe, 0.53 Zn, 0.16 Mn, 0.11 Ni and 0.1 Mg (Lumley, R. N. et al. “Thermal characteristics of heat-treated aluminum high-pressure die-castings” 1 Scripta Materialia 58 (2008) 1006-1009, the entire disclosure of which is incorporated herein by this reference). The developers teach that the Cu-phases, such as the Al2Cu precipitate phase, are important to achieving the benefits of artificial aging, as well as for improving thermal conductivity of the casted part. However the castings suffer from lower corrosion resistance, a high potential for cast defects and a high material cost due to the percentage Cu.
It is known that reducing the Cu content improves the corrosion resistance of an aluminum alloyed material. However Cu is thought to be a necessary hardening component in HDPC aluminum castings. In previously published work, some of the present investigators recommended lower Cu content ranges of 0.5% to 1.5% by weight depending upon the as-cast and heat treatment conditions (see U.S. application Ser. No. 12/827,564, publication No. 20120000578, the entire disclosure of which is incorporated herein by this reference). Nonetheless the presence of Cu in the casting solution after solidification was considered integral to the preservation of acceptable mechanical properties, in particular hardness/yield strength of the cast.
Essentially Cu-free alloys, such as A356, are known in the art, however they are typically used in sand casting and/or semi-permanent mold casting processes other than HDPC and as formulated, suffer from the predicted deficiencies in mechanical properties such as poor tensile strength.
Lin (U.S. patent application Ser. No. 11/031,095) discloses an aluminum alloy having reduced a reduced Cu percentage; however Lin nonetheless teaches the importance of presence of some copper to the hardening process. Moreover, the Lin alloy formulations and castings contain low weight percentages of Si in order to avoid brittle Al—Si eutectic networks in the casted condition. The goal of Lin was to produce aluminum alloys suitable for thixoforming, a molding process which combines features of casting and forging involving low-pressure molding to produce particular microcrystalline structures and to avoid solution heat treatment. The alloys of Lin would be unsuitable for HPDC methods.
Clearly a need exists in the art for an aluminum alloy suitable for HPDC and amenable to age hardening, without compromising corrosion resistance or mechanical properties of the cast components.